Recently, there is a trend toward reduction in size of electric and electronic apparatuses, and a magnetic material is also required to be smaller in size and higher in efficiency. As a conventional magnetic material, for example, there are a ferrite magnetic core using ferrite powder for a choke coil used in a high-frequency circuit and a powder magnetic core that is a metal powder compact.
A ferrite magnetic core is low in saturation magnetic flux density, and poor in direct-current superposing characteristic. Accordingly, in a conventional ferrite magnetic core, there is provided a gap of 200 to 300 μm in a direction vertical to the magnetic path in order to assure direct-current superposing characteristic, thereby preventing the value of inductance L from lowering during direct-current superposition. However, such a wide gap causes a humming noise to be generated, and magnetic flux leakage from the gap causes the winding especially at a high-frequency band to be remarkably increased in copper loss.
On the other hand, a powder magnetic core manufactured by compacting soft magnetic metal powder is far higher in saturation magnetic flux density as compared with ferrite magnetic core, which is therefore advantageous for size reduction. Also, unlike a ferrite magnetic core, it can be used without any gap, and is less in copper loss due to humming noise or magnetic flux leakage.
However, it cannot be said that a powder magnetic core is more excellent than a ferrite magnetic core with respect to permeability and core loss. Particularly, in the case of a powder magnetic core used for a choke coil and inductor, the core is greatly increased in temperature because of remarkable core loss, making it difficult to reduce the size. Also, it is necessary for a powder magnetic core to be increased in compacting density in order to improve its magnetic characteristic, and a compacting pressure of 5 tonne/cm2 or over is usually required in the manufacture. For some products, the compacting pressure required in the manufacture is 10 tonne/cm2 or over. Therefore, it is extremely difficult to manufacture small-sized powder magnetic cores used for choke coils which are mounted in products with complicated shapes such as DC-DC converters for computers and required to be low in height. Accordingly, a powder magnetic core is subjected to greater restrictions as a core shape as compared with a ferrite magnetic core, and it is difficult to reduce the size of the product.
The core loss of powder magnetic core usually consists of hysteresis loss and eddy-current loss. Eddy-current loss increases in proportion to the second power of frequency and to the second power of eddy current flowing size. Accordingly, by coating the surface of metal powder with an insulating material, it is possible to suppress the eddy current flowing size so that it is only within metal powder particles instead of the whole core over metal powder particles. In this way, eddy-current loss can be reduced.
On the other hand, regarding the hysteresis loss, since a powder magnetic core is compacted under a high pressure, considerable strain is introduced into the magnetic material, causing the permeability to be lowered and the hysteresis loss to be increased. In order to avoid this, high-temperature heat treatment is executed for releasing such strain as needed after molding. As for high-temperature heat treatment, an insulative binding agent such as water glass and resin is absolutely needed for insulating and binding the metal powder.
As such a powder magnetic core, conventionally, after the surface of metal powder is coated with tetrahydroxylane (SiOH4), the surface of metal powder is coated with SiO2 through heat treatment. After that, powder magnetic core compacted under pressure and heat-treated and metal powder whose surface is coated with tetrahydroxylane (SiOH4) are subjected to heat treatment to coat the surface with SiO2. After that, synthetic resin as a binding agent is mixed, followed by compacting under pressure and heat treatment, and the powder magnetic core obtained assures binding of metal powder. Such a conventional technology is disclosed in Japanese Patent Laid-Open Application S62-247005 (claims 1 and 2).
FIG. 13 is a conceptual sectional view of powder magnetic core 100 in these conventional examples.
In FIG. 13, reference numeral 101 is metal powder, numeral 102 is SiO2 as an insulating material coated on the surface of metal powder 101, and numeral 103 is synthetic resin as a binding agent filled between metal powder 101.
However, in powder magnetic core 100 thus obtained, SiO2 102 coated on the surface of metal powder 101 is a non-magnetic material, and the existence of a magnetic gap generated between metal powder 101 causes the permeability of powder magnetic core 100 to be lowered. Also, synthetic resin 103 filled between metal powder 101 also turns into a magnetic gap generated between metal powder 101, and in addition, the existence of synthetic resin 103 causes the filling factor of magnetic material in powder magnetic core 100 to be lowered and its permeability to be lowered.
As a core to avoid such lowering of permeability, a powder magnetic core with ferrite being a magnetic material filled between metal powder is conventionally known. Such a powder magnetic core is disclosed in Japanese Patent Laid-Open Application S56-38402.
FIG. 14 is a conceptual sectional view of powder magnetic core 104 in the conventional example. In FIG. 14, reference numeral 105 is metal powder, and numeral 106 is a ferrite layer disposed between metal powder 105.
However, in the case of powder magnetic core 104 in the conventional example wherein ferrite being a magnetic material is filled between metal powder 105, the bonding between metal powder 105 and ferrite layer 106 is not enough to assure sufficient mechanical strength, and there arises a problem of impact resistance. For example, when machining a powder magnetic core, it is finished by a machine at the final stage of machining in order to improve the dimensional accuracy. In that case, there is a problem of cracking in the machining surface or partial peeling and removing.